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. 2025 Mar 27;25(7):2099.
doi: 10.3390/s25072099.

LITES-Based Sensitive CO2 Detection Using 2 μm Diode Laser and Self-Designed 9.5 kHz Quartz Tuning Fork

Junjie Mu  1   2 , Jinfeng Hou  1 , Shaoqi Qiu  3 , Shunda Qiao  1 , Ying He  1 , Yufei Ma  1   2
Affiliations

LITES-Based Sensitive CO2 Detection Using 2 μm Diode Laser and Self-Designed 9.5 kHz Quartz Tuning Fork

Junjie Mu et al. Sensors (Basel). .

Abstract

A carbon dioxide (CO2) sensor based on light-induced thermoelastic spectroscopy (LITES) using a 2 μm diode laser and a self-designed low-frequency trapezoidal-head QTF is reported for the first time in this invited paper. The self-designed trapezoidal-head QTF with a low resonant frequency of 9464.18 Hz and a high quality factor (Q) of 12,133.56 can significantly increase the accumulation time and signal level of the CO2-LITES sensor. A continuous-wave (CW) distributed-feedback (DFB) diode laser is used as the light source, and the strongest absorption line of CO2 located at 2004.01 nm is chosen. A comparison between the standard commercial QTF with the resonant frequency of 32.768 kHz and the self-designed trapezoidal-head QTF is performed. The experimental results show that the CO2-LITES sensor with the self-designed trapezoidal-head QTF has an excellent linear response to CO2 concentration, and its minimum detection limit (MDL) can reach 46.08 ppm (parts per million). When the average time is increased to 100 s based on the Allan variance analysis, the MDL of the sensor can be improved to 3.59 ppm. Compared with the 16.85 ppm of the CO2-LITES sensor with the commercial QTF, the performance is improved by 4.7 times, demonstrating the superiority of the self-designed trapezoidal-head QTF.

Keywords: carbon dioxide (CO2); gas sensing; light-induced thermoelastic spectroscopy (LITES); quartz tuning fork.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
CO2, CO, and H2O absorption lines simulation based on the HITRAN 2023 database. (a) CO2, CO, and H2O absorption line intensity in the range of 4800–5100 cm−1; (b) CO2, CO, and H2O absorption line near 4990 cm−1.
Figure 2
Figure 2
Output characteristic of the used 2 μm diode laser: (a) output laser wavelength varying with injection current and temperature; (b) output power varying with injection current and temperature.
Figure 3
Figure 3
Simulation of stress and surface charge density distribution under frequency-domain excitation: (a) surface charge density simulation of commercial QTF; (b) surface charge density simulation of the self-designed trapezoidal-head QTF; (c) stress simulation of commercial QTF; (d) stress simulation of the self-designed trapezoidal-head QTF.
Figure 4
Figure 4
Temperature difference and temperature gradient variation in LITES simulation: (a) temperature difference in commercial QTF; (b) temperature difference in the self-designed trapezoidal-head QTF; (c) temperature gradient of commercial QTF; (d) temperature gradient of the self-designed trapezoidal-head QTF.
Figure 5
Figure 5
Schematic diagram of the experimental setup for the CO2-LITES sensor.
Figure 6
Figure 6
Frequency response characteristic curves of two types of QTF: (a) commercial QTF; (b) self-designed trapezoidal-head QTF.
Figure 7
Figure 7
The relationship between current modulation depth and signal amplitude: (a) commercial QTF; (b) self-designed trapezoidal-head QTF.
Figure 8
Figure 8
2f signals under different CO2 concentrations: (a) commercial QTF; (b) self-designed trapezoidal-head QTF.
Figure 9
Figure 9
Linear fitting of 2f signal amplitudes under different concentrations of CO2: (a) commercial QTF; (b) self-designed trapezoidal-head QTF.
Figure 10
Figure 10
Allan deviation analysis of CO2-LITES sensor: (a) commercial QTF; (b) self-designed trapezoidal-head QTF.
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